Macroscopically expanded, three-dimensional, apertured polymeric webs are generally known in the art. As utilized herein, the term "macroscopically expanded", when used to describe three-dimensional plastic webs, ribbons and films, refers to webs, ribbons and films which have been caused to conform to the surface of a three-dimensional forming structure so that both surfaces thereof exhibit the three-dimensional pattern of the forming structure, the pattern being readily visible to the naked eye when the perpendicular distance between the viewer's eye and the plane of the web is about 12 inches. By way of contrast, the term "planar", when utilized herein to describe plastic webs, ribbons and films, refers to the overall condition of the web, ribbon or film when viewed by the naked eye on a macroscopic scale. In this context "planar" webs, ribbons and films may include webs, ribbons and films having fine-scale surface aberrations on one or both sides, the surface aberrations not being readily visible to the naked eye when the perpendicular distance between the viewer's eye and the plane of the web is about 12 inches or greater.
One macroscopically expanded, three-dimensional apertured polymeric web which is particularly well suited to transferring fluid deposited on one surface thereof to its opposite surface and thereafter isolating the transferred fluid from a wearer's skin is disclosed in commonly assigned U.S. Pat. No. 3,929,135 issued to Thompson on Dec. 30, 1975, the disclosure of which is incorporated by reference herein. Thompson describes a macroscopically expanded, three dimensional web (e.g., a topsheet) comprised of liquid impermeable material, but provided with a pattern of tapered capillaries, the capillaries having a base opening in the plane of the topsheet and an apex opening remote from the plane of the topsheet, the apex opening being in intimate contact with the absorbent pad utilized in the disposable absorbent bandage. The Thompson topsheet allows the free transfer of fluids from the wearer's body into the absorbent element of the device while inhibiting the reverse flow of these fluids. This provides a relatively much drier surface in contact with the user than had previously been obtainable.
Another macroscopically expanded, three-dimensional apertured plastic web well suited for use as a topsheet on absorbent bandages such as sanitary napkins is disclosed in commonly assigned U.S. Pat. No. 4,342,314 issued to Radel and Thompson on Aug. 3, 1982, the patent being hereby incorporated by reference herein. The macroscopically expanded, three-dimensional plastic web disclosed in the Radel and Thompson patent exhibits a fiber-like appearance and tactile impression which has been favorably received by consumers when used as a wearer contacting surface.
According to the teachings of the aforementioned commonly assigned patents to Thompson and to Radel, et al., plastic webs of the aforementioned type can be made by applying a fluid pressure differential to the web while it is supported on a three-dimensional forming structure until the web is macroscopically expanded to comply with the three-dimensional cross-section of the forming structure on which if is supported. When aperturing of the macroscopically expanded, three-dimensional web is desired, the fluid pressure differential is applied continuously until such time as aperturing of the web in areas coinciding with the apertures in the forming structure has been completed.
While single-phase (i.e., one aperturing phase) forming processes (including the fluid-based systems discussed above and the vacuum-based systems discussed below) have been successfully utilized in producing macroscopically expanded, three-dimensional apertured plastic webs exhibiting many characteristics generally viewed as favorable by consumers, such single-phase processing techniques are unable to deliver certain desired characteristics in a single finished web structure, particularly at high production speeds.
In this regard, a multi-phase, fluid-based process such as that described in U.S. Pat. No. 4,609,518, issued Sep. 2, 1986 to Curro et al. (hereafter referred to as "'518 patent"), was developed to provide a film with very small and very large apertures immediately adjacent one another. As the patent discloses, the formation of the very small (including micro-sized) apertures in the direction opposite those formed for large apertures hinders the ability of initially unabsorbed fluid from running off the web's surface. Thus, fluid not immediately transported through the large apertures is restrained from running off the web's surface, and is subsequently taken up through the larger apertures and is deposited in the article's core where the web is used as a topsheet material. These outwardly formed small apertures also reduce the level of web/skin contact and reduce the rigidity of the film, and thereby feel more comfortable to the user. Alternatively, the '518 patent discloses films where the very small apertures are formed in the same direction as the macro-apertures.
An alternative to forming apertured, three-dimensional films via use of fluid pressure differentials is the use of vacuum-forming such structures. For example, U.S. Pat. No. 3,054,148 issued Sep. 18, 1961 to Zimmerli describes a perforated film material formed by heating a film on a perforated screen while applying sufficient vacuum to the underside of the screen to form apertures in the film. The use of vacuum formation is limited in that formation of apertures in two directions, such as is described in the '518 patent, is not possible because heating the apertured film for the requisite second pass damages the apertures formed in the first pass. As such, the tactile/feel benefits obtainable using that process are not obtainable using vacuum perforation methods. In addition, micro-sized apertures obtainable using fluid formation (e.g., the '518 patent) is generally not obtainable using vacuum forming processes because typical vacuum power applied in such processes is insufficient to rupture the film to form microsized apertures.
Regardless of the means employed for aperture formation, where a wettable film material is desired, the above references generally obtain such a structure by surface treating the naturally hydrophobic polymeric web with a wetting agent. Surface treatment is generally accomplished by either spraying surfactant onto the web's surface or by dipping the web in a surfactant-containing bath. Regardless of which of these methods is employed, surface treatment suffers from the inability to precisely control the location and level of treatment, as well as adverse effects caused by migration of significant amounts of surfactant into apertures and other components (e.g., absorbent core) when the apertured film is used as a topsheet in an absorbent article. Surface treatment further suffers from the disadvantage that desirable wetting agents, or surfactants, tend to be washed off upon repeated exposure to such fluids. Thus, when used as a topsheet in an absorbent article, the treated films lose their ability to transport fluid away from the skin and into the article's core after repeated wettings.
U.S. Pat. No. 4,535,020, issued to Thomas et al. on Aug. 13, 1985, addressed some of the problems associated with surface treating vacuum-formed apertured films by incorporating hydrophilic surfactant in the polymeric resin before extrusion for film formation. After extrusion of the resin/surfactant mixture, and subsequent vacuum formation of the apertures, the incompatible surfactant eventually blooms to the film's surface to provide a more durably wettable web. As indicated above, however, the use of vacuum formation means for aperturing films has inherent limitations relative to the use of fluid formation means.
In this regard, co-assigned U.S. Pat. No. 5,520,875, issued May 28, 1996 to A. J. Wnuk et al., addresses the problem of surfactant wash-off where apertures are formed using a fluid pressure differential by co-extruding multiple polymeric film layers, referred to therein as core layers and outer layers. The outer layer, which is exposed to the aperture-forming fluid, is free of surfactant, while the core layer contains a migratable surfactant. After co-extrusion and aperture formation, the surfactant migrates from the core layer to the outer layer's surface, to provide a durably wettable multilayer web. As such, the patent addresses many of the undesired aspects of prior aperturing processes. However, this process is clearly limited to the production of multilayer films. Thus, the patent describes a process that requires additional raw materials that may serve no function other than avoiding surfactant wash-off during manufacture, thereby potentially adding undesired caliper, complexity and/or cost when the laminate is used as an absorbent article topsheet. Similarly, the patent requires co-extrusion of separate layers of material, which may add complexity to the formation process and formation equipment.
In spite of the teachings of the prior art, there remains a need for a single-layer, apertured film material that offers both durable wettability and a soft feel to the skin. There also remains the need for a process that provides a single-layer, apertured film having improved durable wettability, wherein the process minimizes wash-off of surfactant during aperture formation using a fluid pressure differential. There also remains the need for a multi-phase process for producing a single-layer, apertured film having improved durable wettability and soft feel to the skin.
Accordingly, it is an object of the present invention to provide a process wherein various combinations of previously incompatible characteristics can be provided in a single-layer, apertured polymeric web.
It is another object of the present invention to provide a multi-phase process for producing durably wettable, apertured plastic webs wherein the different phases of the process may be separated from one another either temporally or spatially or both.
It is still another object of the present invention to provide apertured plastic webs which offer improved durable wettability along with highly preferred appearance, softness and tactile impression.
It is still another object of the present invention to provide a process for fluid aperturing a continuous polymeric film wherein the process minimizes surfactant wash-off. Minimizing surfactant wash-off provides two distinct benefits. First, reduced levels of surfactant are required to obtain a wettable web. Second, reducing the level of washed-off surfactant during film processing prevents undesired foaming and alleviates the adverse effect surfactants have on machine performance after continuous exposure.